1. Field of the Invention
The present invention relates to voltage step-up charge pumps and more specifically to a regulation system of such a charge pump.
2. Discussion of the Related Art
A charge pump is often used to provide a voltage raised with respect to the supply voltage. Such an overvoltage is for example used to turn on an N-channel MOS transistor, the drain of which is connected to the supply voltage, this, to obtain on the transistor source a voltage equal to the supply voltage. Such a transistor is often used in a DRAM to charge the memory cells to the supply voltage.
FIG. 1 shows a charge pump 10 coupled with a conventional regulation system 20. Charge pump 10 generates an overvoltage VPP.
Regulation system 20 includes a circuit 30 generating a reference voltage Vref on a terminal 31, and a measurement circuit 40 generating a measurement voltage Vmes on a terminal 41. It further includes a comparator 50, the inputs of which are connected to terminals 31 and 41 and which generates on a terminal 51 a charge pump enable signal.
Reference circuit 30 includes an N-channel MOS transistor 32, diode-connected between a supply voltage terminal Vdd and terminal 31 and a plurality 33 of N-channel MOS transistors, series-connected between terminal 31 and the ground, all controlled by a same bias voltage BIASN. Transistors 33 form a resistor of relatively high value. It should be noted that the voltage of terminal 31 does not vary as long as Vdd does not vary, and is used as a reference voltage Vref.
Measurement circuit 40 includes a first N-channel MOS transistor 42, diode-connected between terminal 41 and the supply voltage terminal via a second N-channel MOS transistor 43. The gate of transistor 43 receives overvoltage VPP generated by charge pump 10. Measurement circuit 40 further includes a plurality 44 of N-channel MOS transistors, series-connected between terminal 41 and the ground, controlled by bias voltage BIASN.
With this configuration, voltage VPP is regulated to Vdd+Vt, where Vt is the threshold voltage of transistor 43. Indeed, the source voltage of transistor 43 settles at VPP-Vt. If voltage VPP-Vt is smaller than Vdd, voltage Vmes is smaller than voltage Vref, whereby comparator 50 activates charge pump 10 to increase voltage VPP. Voltage VPP increases until voltage VPP-Vt on the source of transistor 43 is equal to Vdd, in which case voltage Vmes is equal to Vref and comparator 50 deactivates charge pump 10.
Clearly, transistor 43 and transistors of charge pump 10 are submitted to a voltage VPP greater than supply voltage Vdd. Supply voltage Vdd is allowed to vary within a given range under the voltage allowed by the technology used, with overvoltage VPP of course having to be smaller than this allowable voltage. Now, with recent technologies, the overvoltage VPP that would be obtained if the supply voltage were equal to its upper limit is so close to the allowable voltage that the transistors submitted to voltage VPP would end up being damaged in time.